Carbon black for tire tread rubber

ABSTRACT

There is disclosed carbon black for a tire tread rubber which has a nitrogen adsorption specific surface area (N 2  SA) of 60 to 160 m 2  /g and a dibutyl phthalate absorption (DBP) of 90 to 150 ml/100 g which belong to the respective regions of hard grades of carbon black, and an intraaggregate void volume Vp (ml/g) at least equal to the value calculated according to the equation: [0.00976×DBP-0.0358].

This is a continuation of Application No. 373,973, filed Jun. 30, 1989,now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to carbon black for a tire tread rubber,and more particularly to carbon black which can impart a high abrasionresistance to a tire tread rubber while keeping the heat build-up in thetire tread rubber on a conventional level.

The effect of reinforcing a rubber by carbon black has heretofore beenconsidered to be largely governed by the specific surface area (particlesize) and structure of carbon black. Accordingly, there are known manygrades of carbon black differing in these properties.

In compounding carbon black into a rubber component, an appropriatechoice is made of the grade of carbon black having characteristicsadapted to the application of a rubber composition to be prepared. Forexample, a hard grade of carbon black, such as N110 or N220, is used ina rubber member requiring a high abrasion resistance, such as a tiretread portion subject to severe running conditions. As the runningconditions for tires have recently become more and more severe, however,such a high performance has been required of a tire tread portion so asto satisfy particularly a high abrasion resistance and a low heatbuild-up at the same time.

In general, it is known that the abrasion resistance of a tire treadportion is enhanced as the specific surface area and structure of carbonblack compounded thereinto are increased. However, it also is known thatthe heat build-up in the tire tread portion is increased in keeping withincreases in the specific surface area and structure of carbon black.Thus, the abrasion resistance has an antinomic relation with the heatbuild-up. Accordingly, it has been considered extremely difficult tosimultaneously impart a high abrasion resistance and a low heat build-upto a rubber composition.

With a view to solving such a difficulty, there was proposed an attemptto impart a high abrasion resistance and a low heat build-up to a tiretread by using furnace carbon black having a nitrogen adsorptionspecific surface area (N₂ SA) of 94 to 145 m² /g, a dibutyl phthalateabsorption of 89 to 107 ml/100 g, and a true specific gravity lower thanthe value calculated according to the equation: {(-0.0006×N₂ SA (m²/g)+1.8379} (see Japanese Patent Publication No. 34,149/1978). Therealso was proposed an attempt to simultaneously impart a high abrasionresistance and a low heat build-up to a tire tread by using carbon blacksatisfying the following three conditions: N₂ SA=100˜200 m² /g T=ΔDst⁵⁰×ΔDst⁷⁵ .R, and 4,100-18.1 (N₂ SA)<T<7,700-25.5 (N₂ SA), wherein ΔDst isa difference between two equivalent Stokes diameters and R is themaximum absorbance at a Stokes mode diameter, to improve a workabilityin dispersing it into a rubber (see Japanese Patent Laid-Open No.112,638/1988). Despite such proposals, however, no rubber compositionscontaining, compounded thereinto, carbon black having such specifiedproperties can simultaneously secure the satisfactory levels of abrasionresistance and heat build-up, in which further improvements hastherefore been demanded.

In view of the above, the inventors of the present invention have madeinvestigations on not only the N₂ SA and dibutyl phthalate absorption ofcarbon black but also the intraaggregate void volume thereof, and havefound that, when carbon black in the form of aggregates having a certainlevel of structure and a large intraaggregate void volume is compoundedinto a rubber component, the resulting rubber composition has a highabrasion resistance and a low heat build-up. The present invention hasbeen completed based on this finding.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide carbon black whichcan impart a high abrasion resistance to a tire tread rubber whilekeeping the heat build-up in the tire tread rubber on a conventionallevel.

A second object of the present invention is to provide carbon blackwhich can be suitably used in a tread rubber of large tires for trucks,buses, etc. to be put into service under severe running conditions.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an example of a reactor tobe used in the production of the carbon black of the present invention,and

FIG. 2 is an enlarged cross-sectional view of the essential portion ofthe reactor shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The carbon black for a tire tread rubber according to the presentinvention has a nitrogen adsorption specific surface area (N₂ SA) of 60to 160 m² /g and a dibutyl phthalate absorption (DBP) of 90 to 150ml/100 g which belong to the respective regions of hard grades of carbonblack, and an intraaggregate void volume Vp (ml/g) at least equal to thevalue calculated according to the equation: (0.00976×DBP-0.0358). Amongthe above-mentioned characteristic properties, the nitrogen adsorptionspecific surface area (N₂ SA) of 60 to 160 m² /g and the dibutylphthalate absorption of 90 to 150 ml/100 g, which are a particle-relatedrange and a structure-related range, respectively, belonging to therespective regions of hard grades of carbon black, are prerequisites forthe carbon black to impart a high abrasion resistance to a compoundingrubber. When the nitrogen adsorption specific surface area (N₂ SA) issmaller than 60 m² /g, or when the dibutyl phthalate absorption is lowerthan 90 ml/100 g, carbon black cannot impart a high abrasion resistanceto a tire tread. When the nitrogen adsorption specific surface area (N₂SA) exceeds 160 m² /g, the dispersibility of carbon black in a rubbercomponent is reduced to thereby inhibit an improvement in the abrasionresistance thereof and bring about a high heat build-up. When thedibutyl phthalate absorption exceeds 150 ml/100 g, carbon blackunfavorably lowers the ice skid performance of a tire tread.

The value calculated according to the equation: [0.00976×DBP-0.0358],which is the lower limit of the intraaggregate void volume Vp (ml/g) ofthe carbon black of the present invention, assures that the carbon blackof the present invention has an aggregate texture peculiar to a highlydeveloped structure which has a high anisotropy as compared with thoseof conventional grades of carbon black to be compounded into a rubbercomponent. When the intraaggregate void volume Vp (ml/g) is equal to orlarger than the value calculated according to the above-mentionedequation, carbon black can impart a superb abrasion resistance to a tiretread while securing a low heat build-up therein comparable to those intire treads containing, compounded thereinto, a conventional grade ofcarbon black.

Carbon black having a large intraaggregate void volume Vp essentiallyhas an aggregate texture with a developed structure, which works toenhance the reinforcement of a rubber when the carbon black iscompounded into the rubber. The carbon black of the present inventionhaving an intraaggregate void volume Vp at least equal to the valuecalculated according to the equation: (0.00976×DBP-0.0358) isconspicuously developed in anisotropy. The developed anisotropy servesto remarkably improve the abrasion resistance of a rubber component. Thenitrogen adsorption specific surface area (N₂ SA) of 60 to 160 m² /g andthe dibutyl phthalate absorption of 90 to 150 ml/100 g, which areprerequisites for carbon black to impart a high abrasion resistance to arubber as described above, may serve synergistically with theabove-mentioned intraaggregate void volume Vp to provide a high abrasionresistance to a rubber component while securing a conventional level ofheat build-up therein.

The characteristic values of the carbon black of this invention weremeasured according to the following methods

(1) Nitrogen adsorption specific surface area (N₂ SA): ASTM D 3037-78"Standard Methods of Testing Carbon Black--Surface Area by NitrogenAdsorption" Method B. The N₂ SA of IRB No. 5 measured according to thismethod was 80.3 m² /g (IRB stands for Industry Reference Black.)

(2) Dibutyl phthalate absorption (DBP): JIS K6221 (1975) "Method ofTesting Carbon Black for Rubber", Section 6.1.2, Method A (correspondingto ASTM D2414-82)

A prescribed quantity of dry carbon black is placed in the mixingchamber of an adsorptometer. Dibutyl phthalate is added dropwise to thecarbon black from a buret with mixing. The buret is closed automaticallyby the action of a limit switch when the torque of the rotor in themixing chamber reaches a predetermined value. The absorption iscalculated from the following equation. ##EQU1## wherein DBP: absorptionof dibutyl phthalate (ml/100 g) V: volume of dibutyl phthalate added(ml)

W_(D) : quantity of dry carbon black (g)

(3) Intraaggregate void volume Vp (ml/g)

Using a mercury porosimeter Poresizer 9300 manufactured byMicromeritics, a carbon black sample is immersed in mercury, to which aslowly increasing pressure is applied to gradually infiltrate themercury into the micropores of the carbon black in accordance with thepressure. The intraaggregate void volume is calculated from therelationship between the pressure and the amount of mercury infiltratedaccording to the equation (1). ##EQU2## wherein X₁ : reading of themercury porosimeter at 25 psi X₂ : reading of the mercury porosimeter at30,000 psi

W: weight of carbon black sample (g)

CF: constant determined by a cell used in the measurement

Additionally stated, the intraaggregate void size corresponding to theapplied pressure of 25 psi is 7.2 μm, while that corresponding to theapplied pressure of 30,000 psi is 0.006 μm.

The carbon black of the present invention is produced using, forexample, a Y-shaped oil furnace as shown in FIG. 1 (see Japanese PatentPublication No. 10,581/1987). This oil furnace comprises two generators1 and 1' and a main reaction zone 2 extending from a position where thetwo generators converge. Each generator is made up of a wind box 4, aburner 5, a combustion chamber 7 having therein a feedstock oil spraynozzle 6, and a pyrolysis duct 8 integrated with the combustion chamber7. The hydrocarbon feedstock oil is sprayed into the combustion gas offuel oil via the feedstock oil spray nozzle 6 so that the oil spray ispyrolyzed to form a gas stream of carbon black intermediate. The two gasstreams of carbon black intermediate are simultaneously entrained intothe reaction chamber 2 at a high speed to impinge against each other atpoint P in a space 9. Thereafter, the resulting stream is cooled withwater sprayed at the position of a cooling water spray 3 and carbonblack is then separated therefrom. The conditions of forming the gasstreams of carbon black intermediate in the generators 1 and 1' arecontrolled to adjust the intraaggregate void volume Vp of the resultingcarbon black, while the conditions of burning in the furnace, theresidence time in the furnace of the stream of carbon black beingproduced, etc. are controlled to adjust the nitrogen adsorption specificsurface area (N₂ SA) and dibutyl phthalate absorption thereof. In theforegoing manner, carbon black having the characteristic propertiesspecified in the present invention can be produced.

As described above, the carbon black of the present invention which hasa highly anisotropic aggregate texture with a highly developed structurecan impart a high abrasion resistance to a compounding rubber whilesecuring a conventional level of heat build-up therein. Accordingly, thecarbon black of the present invention can be suitably used in a treadrubber of large tires for trucks, buses, etc. to be put into serviceunder severe running conditions.

According to a customary method, the carbon black of the presentinvention is compounded into an elastomer, examples of which include anatural rubber, a styrene-butadiene rubber, a polybutadiene rubber, anisoprene rubber, a butyl rubber, and other various synthetic rubbers andmixed rubbers capable of being reinforced with common carbon black.

35 to 100 parts by weight of the carbon black of the present inventionis compounded into 100 parts by weight of a rubber component. The carbonblack and the rubber component are kneaded together with other necessarycomponents such as a vulcanizing agent, a vulcanization accelerator, anage resister, a vulcanization aid, a softener, and a plasticizer toprepare a rubber composition for tire treads.

Examples of the present invention will now be described in comparisonwith Comparative Examples.

The methods of measuring various characteristic properties of vulcanizedrubber compositions in Examples and Comparative Examples are as follows.(a) Abrasion Loss

Abrasion loss was measured with a Lambourne abrasion tester (withmechanical slip mechanism) under the following conditions:

test piece: 10 mm thick, 44 mm in outside diameter

Emery wheel GC type; grain size: #10; hardness: H

carborundum added: grain size: #80, adding rate: approximately 9 g/min

relative slip ratio of Emery wheel surface to test piece: 24%, 60%

speed of revolution of test piece: 535 rpm

(b) Hysteresis Loss (tan δ)

Hysteresis loss was measured with a viscoelastic spectrometer(manufactured by Iwamoto Seisakusho Co.) under the following conditions:

test piece: 2 mm thick, 30 mm long, 5 mm wide

temperature: room temperature

frequency: 50 Hz

dynamic strain (amplitude): ±1%

(c) Other Properties

All other measurements were made according to JIS K6301 "Physical TestMethod for General Rubbers"

EXAMPLE 1 Proproduction of Carbon Black

The oil furnace used has a Y-shaped structure as shown in FIG. 1 whichcomprises two generators 1 and 1' so arranged as to converge at an angleof 60° with each other in front of a main reaction zone 2 having a frontnarrow portion 9 of 90 mm in inside diameter and 700 mm in length and arear broad portion 10 of 200 mm in inside diameter and 2,000 mm inlength. Each generator comprises a pyrolysis duct 8 (60 mm in insidediameter and 500 mm long) and a combustion chamber 7 (400 mm in insidediameter and 800 mm long, including 200 mm of conical sections) providedwith a burner 5 and a feedstock oil spray nozzle 6 arranged coaxiallywith each other through a wind box 4 provided around a front portionthereof. A ring member 11 having a constriction ratio of 0.65 isprovided 50 mm downstream of the intersectional point P in the frontnarrow portion 9 as shown in FIG. 2. The ring member 11 is made of afire brick. The constriction ratio m is expressed by the followingequation: ##EQU3## wherein D=90 mm and D₀ =72.5 mm.

The feedstock oil used was an aromatic hydrocarbon oil having a specificgravity (15/4° C.) of 1.0703, an Engler viscosity (40/20° C.) of 2.10, abenzene-insolubles content of 0.03%, a correlation index (BMCI) of 140and an initial boiling point of 103° C. The fuel oil used was ahydrocarbon oil having a specific gravity (15/4° C.) of 0.903, aviscosity (at 50° C.) of 16.1 cSt, a residual carbon content of 5.4%, asulfur content of 1.8% and a flash point of 96° C.

Three kinds of carbon black (Runs Nos. 1 to 3) according to the presentinvention were produced from the above-mentioned feedstock oil using theabove-mentioned oil furnace and fuel oil under conditions as listed inthe following Table 1.

                  TABLE I                                                         ______________________________________                                                              Fuel  Fuel   Feedstock                                                                             Resi-                                           Total air                                                                              oil feed                                                                            combus-                                                                              oil feed                                                                              dence                              Run  Gener-  feed rate                                                                              rate  tion rate                                                                            rate    time                               No.  ator    (Nm.sup.3 /H)                                                                          (kg/H)                                                                              (%)    (kg/H)  (msec)                             ______________________________________                                        1    1       200      11.6  160    46.4    5.2                                     1'      300      17.3  160    57.9                                       2    1       250      15.4  150    41.6    4.0                                     1'      350      21.6  150    46.2                                       3    1       200      12.3  150    41.2    5.0                                     1'      330      20.3  150    54.9                                       ______________________________________                                    

Table II shows the data on the three kinds of carbon black thus producedwith respect to nitrogen adsorption specific surface area (N₂ SA). DBPabsorption, intraaggregate void volume Vp, and value calculatedaccording to the equation: [0.00976×DBP-0.0358].

The data on Runs Nos. 4 to 7 in Table II which are listed as comparativeexamples are those of hard grades of carbon black produced byconventional techniques, which have a nitrogen adsorption specificsurface area (N₂ SA) of at least 60 m² /g but an intraaggregate voidvolume Vp falling outside the range specified in the present invention.

                  TABLE II                                                        ______________________________________                                               Ex.                                                                           Ex.        Comp. Ex.                                                          Run No.                                                                         1      2      3    4     5     6     7                               Properties                  (N330)                                                                              (N220)                                                                              (N110)                                ______________________________________                                        N.sub.2 SA (m.sup.2 /g)                                                                112    143    120   76   117   143   152                             DBP      109    145    115  106   118   116   143                             (ml/100 g)                                                                    Vp (ml/g)                                                                              1.11   1.45   1.20 0.95  1.07  1.05  1.30                            value of 1.03   1.38   1.09 1.00  1.12  1.10  1.36                            equation                                                                      ______________________________________                                    

EXAMPLE 2

Each of the three kinds of carbon black produced in Example 1 wasblended with natural rubber and other components at a blending ratio asshown in Table III.

                  TABLE III                                                       ______________________________________                                        Components          parts by weight                                           ______________________________________                                        natural rubber (RSS #1)                                                                           100                                                       carbon black        50                                                        aromatic oil (softener)                                                                           4                                                         stearic acid        3                                                         (dispersing vulcanization aid)                                                zinc oxide (vulcanization aid)                                                                    5                                                         dibenzothiazyl disulfide                                                                          1                                                         (vulcanization accelerator)                                                   sulfur (vulcanizing agent)                                                                          2.5                                                     ______________________________________                                    

The compound as shown in Table 3 was vulcanized at a temperature of 145°C. for 40 minutes to prepare a rubber composition, which was thenexamined with respect to various rubber characteristics. The results areshown in Table IV with the same Runs Nos. as those of carbon black inTable II.

                                      TABLE IV                                    __________________________________________________________________________              Ex.                                                                           Ex.         Comp. Ex.                                                         Run No.                                                             Properties                                                                              1   2   3   4   5   6   7                                           __________________________________________________________________________    Hardness (JIS Hs)                                                                        64  66  65  63  63  65  66                                         300% Modulus                                                                            126 132 129 112 120 115 130                                         (kg/cm.sup.2)                                                                 Tensile strength                                                                        298 290 296 274 286 289 287                                         (kg/cm.sup.2)                                                                 Elongation (%)                                                                          570 530 560 580 570 595 535                                         Abration loss                                                                 slip ratio 24%                                                                          0.0702                                                                            0.0649                                                                            0.0660                                                                            0.0892                                                                            0.0747                                                                            0.0710                                                                            0.0684                                      slip ratio 60%                                                                          0.0957                                                                            0.0831                                                                            0.0874                                                                            0.1204                                                                            0.1011                                                                            0.0966                                                                            0.0880                                      Hysteresis loss                                                                         0.261                                                                             0.275                                                                             0.265                                                                             0.244                                                                             0.270                                                                             0.271                                                                             0.282                                       (tan δ)                                                                 __________________________________________________________________________

As is apparent from the results of Table IV, the rubber compositions ofExamples according to the present invention were improved in abrasionresistance per hysteresis loss (tan δ) while securing substantially thesame level of hysteresis loss (tan δ) as those of Comparative Exampleswherein use was made of substantially the same grade of conventionalcarbon black. The hysteresis loss is an indicator of heat build-up.

What is claimed is:
 1. Carbon black for a tire tread rubber which has anitrogen adsorption specific surface area (N₂ SA) of 60 to 160 m² /g anda dibutyl phthalate absorption (DBP) of 90 to 150 ml/100 g and anintraaggregate void volume Vp (ml/g) at least equal to the valuecalculated according to the equation: (0.00976×DBP-0.0358).